The present invention relates generally to plasma etch systems and methods, and more particularly, to systems and methods for plasma etch having independent control of ion generation and dissociation of process gas.
A simplified description of a typical plasma etching process is plasma chamber with a process gas therein. The process is excited by an RF or microwave signal electrically coupled into the process gas. Exciting the process gas causes ions and radicals to form within the process gas. The ions and radicals are then directed toward a surface to be etched. The ions and radicals can be directed toward the surface to be etched using gas flows and electrical biasing of various surfaces within the plasma chamber. The ions and radicals react with the material in the surface to be etched.
Increasing a density of an etching plasma is one method to increase how fast the etch surface is etched (e.g., etch rate, usually expressed in Angstroms per minute). Increasing the density of an etching plasma increases the concentration of the ions and thus increases the reactivity between the process gas, ions and the etch surface. However, increasing plasma density can also cause excess dissociation of the process gas mixture into constituent elements or molecules, beyond the level of dissociation which is optimal.
For example, increased plasma density typically causes atomic fluorine radicals to dissociate from a fluorocarbon process gas mixture. The dissociated fluorine may accelerate the undesirable etch of the etch mask, of the feature sidewall, or of the etch stop layer underlying the layer to be etched. Additionally, the excessive dissociation of fluorocarbon source gas may lead to non-optimal composition of fluorocarbon radical species in the plasma, with effects similar to excessive fluorine.
There are other issues of uniformity and control that are also reduced or less than desirable that can occur when the process gas is dissociated too much. In view of the foregoing, there is a need for systems and methods for plasma etch having independent control of ion generation and dissociation of process gas.